When a vehicle turns a corner, an inner drive wheel and an outer drive wheel of the vehicle have to be allowed to be rotated at respective different rotational speeds. The vehicle is provided with a differential device including: a differential casing; a pinion gear which is fixed to a final gear and which is rotatably held in the differential casing; and a pair of side gears which mesh with the pinion gear and which are connected to respective axles each transmitting a drive force to a corresponding one of the drive wheels, so that, upon turning of the vehicle, the differential device is configured to distribute the drive force to the axles corresponding to the right and left drive wheels while allowing a difference between the rotational speeds of the respective axles.
In the above-described differential device, during straight running of the vehicle, there is no difference between rotational speeds of the pair of side gears, namely, a differential rotation is not caused. That is, during the straight running, the pinion gear is not rotated about its axis (although the pinion gear is revolved about axes of the axles together with rotation of the differential casing), and each of the side gears and the differential casing are not rotated relative to each other, without difference of the rotational speed therebetween. However, when the vehicle turns a corner, a resistance applied to one of the side gears that drives the vehicle through the inner drive wheel is made larger than a resistance applied to the other of the side gears that drives the vehicle through the outer drive wheel, whereby a rotational speed of the above-described one of the side gears is relatively reduced while a rotational speed of the above-described other of the side gears is relatively increased. This differential rotation of the side gears causes the pinion gear to be rotated about its axis, and causes the rotational speed difference between each of the side gears and the differential casing. The rotational speed difference could cause wears of the washer (interposed between each side gear and the differential casing), each side gear and the differential casing. For reducing the wears of the washer, side gear and differential casing, there is known an arrangement in which the washer is given an improved wear resistance, for example, by providing a surface of the washer with a surface treatment or a coating that improves the wear resistance. JP-H08-247261A discloses an arrangement for reducing the wears by lubricant supplied to a lubricant passage that is provided in a contact surface of the differential casing which is contact with the washer interposed between the differential casing and each side gear, such that the lubricant passage radially passes through portions of the differential casing that are opposed to respective outer and inner peripheries of the washer.